Substitutional effects of Ni on thermoelectric properties of cobalt spinels to yield more than 10-fold increase in the thermoelectric figure of merit

Abstract

The low thermoelectric figure of merit, ZT, is severely limiting the wide practical applications of thermoelectric materials. In this research theoretical calculations were performed on Ni substituted cobalt oxide (Co3O4) semiconductor materials to identify the qualitative trend of thermoelectric properties due to substitutions. Ni substitutions were made in the tetrahedral sites of 56 atom Co3O4 super cell. With the increasing Ni substitution, the band gap near the Fermi region started to close, from 0.337 eV for pure Co3O4 to 0 eV with Ni+2 substitutions in the tetrahedral sites of Co3O4, thereby showing an increasing metallic behavior which is also evident from the electrical conductance calculations. To calculate band structure, Seebeck coefficient, electrical conductance and thermal conductance due to electrons, first principles based density functional theory, DFT, combined with non-equilibrium Green's function method for two-probe systems were used. To calculate the lattice thermal conductance, reverse non-equilibrium molecular dynamics was used. The calculated Seebeck coefficients were 720 μV/K for pure Co3O4 and decreased exponentially to 35 μV/K for Co2NiO4, where the qualitative trend aligned very well with the available experimental data for Ni substituted cobalt oxide materials. These calculated properties assisted in calculating the theoretical thermoelectric figure of merit, and to qualitatively assess the effect of doping on the overall performance. Improved thermoelectric properties such as low thermal conductance and high Seebeck coefficient for low Nickel substitutions (12.5% of tetrahedral sites represented with Ni0·125Co2·875O4), was observed which yielded a 10-fold increase in thermoelectric figure of merit, ZT, in comparison to pure Co3O4 at 300 K. Additionally, the effect of Nickel substitutions on the p-type behavior was presented and discussed in this research article.